1. Field of the Invention
The present invention relates to non-aqueous electrochemical cells and, more particularly, to improvements with respect to the recharging of secondary active metal cells including those utilizing anodes of lithium metal. The invention allows such cells to achieve suitable high recharging efficiencies without sacrifice of high cell capacity.
2. Description Of the Prior Art
As is well known in the art, non-aqueous active metal cells have enabled those skilled in the art to achieve much higher energy densities or energy to weight ratios then was heretofore known. The potential uses for such cells has created a great deal of interest, resulting in significant efforts to improve the safety and versatility of these cells.
Such cells typically consist of a light, strongly reducing anode, normally an alkali metal such as lithium, an aprotic, non-aqueous solvent into which has been dissolved an appropriate quantity of a salt of the anode metal to form a conductive solution, and an oxidizing agent as the cathode material. In some instances, the electrolyte solvent or co-solvent also acts as the active cathode material which is subsequently reduced at a porous carbon electrode. Such dual-function solvents include thionyl chloride, sulfuryl chloride, and sulfur dioxide.
While primary lithium cells have been in use for quite some time, the development of a practical rechargeable lithium cell has been severely hindered by the inability to efficiently recharge the lithium anode. Because of its high reactivity, a significant fraction of the lithium metal plated out during recharging undergoes side reactions with the electrolyte solution and thereby becomes unsheathed in a layer of nonconductive reaction products. This encapsulated lithium is electrically insulated from the anode and thus can no longer participate in the electrochemical reactions. Some lithium is lost in this manner during each recharge cycle until virtually the whole anode is consumed. The lithium cycling efficiency provides a measure of the rechargeability of the lithium anode in a given electrolyte solution and values of greater than 90%, and preferably greater than 95%, are needed if rechargeable lithium cells are to be practical.
In the prior art, most rechargeable lithium cells employed an organic solvent-based electrolyte solution with an insoluble, solid oxidant cathode material. Ether-based electrolyte solutions have been predominately used in these cells. In particular, cells utilizing LiAsF.sub.6 dissolved in 2-methyl tetrahydrofuran (2-methyl THF) as the electrolyte solution have provided the best overall performance, yielding lithium cycling efficiencies of up to 97% in prototype hardware cells. However, while these solutions are very promising with respect to lithium cycling efficiency, they exhibit low conductivities which severely degrade cell performance at low temperatures (i.e. below 0.degree. C. ) or at high rates of discharge.
Thus, there remains a need in the art for secondary rechargeable cells exhibiting both of the desirable characteristics including:
1. High lithium cycling efficiencies so that a practical number of cycles can be achieved during the life of the cell and,
2. sufficient cell conductivity so that the advantage obtained by the high energy density nature of the cell can be maintained over a broad temperature interval and over a wide range of discharge rates.